Rework makefile in something closer to upstream Makefile to ease maintainance

lib/libmandoc/Makefile

@@ -7,11 +7,34 @@ LIB=	mandoc
 #NO_PIC=
 INTERNALLIB=
 MAN=	mandoc.3
-SRCS=	mdoc_argv.c mdoc_hash.c mdoc_macro.c mdoc_validate.c \
-	mdoc.c arch.c att.c lib.c st.c vol.c \
-	man.c man_hash.c man_macro.c man_validate.c \
-	roff.c eqn.c tbl.c tbl_data.c tbl_layout.c tbl_opts.c \
-	mandoc.c mandoc_aux.c chars.c msec.c read.c
+LIBMAN_SRCS=	man.c \
+		man_hash.c \
+		man_macro.c \
+		man_validate.c
+LIBMDOC_SRCS=	arch.c \
+		att.c \
+		lib.c \
+		mdoc.c \
+		mdoc_argv.c \
+		mdoc_hash.c \
+		mdoc_macro.c \
+		mdoc_validate.c \
+		st.c \
+		vol.c
+LIBROFF_SRCS=	eqn.c \
+		roff.c \
+		tbl.c \
+		tbl_data.c \
+		tbl_layout.c \
+		tbl_opts.c
+SRCS=		${LIBMAN_SRCS} \
+		${LIBMDOC_SRCS} \
+		${LIBROFF_SRCS} \
+		chars.c \
+		mandoc.c \
+		mandoc_aux.c \
+		msec.c \
+		read.c
 
 WARNS?=	3
 CFLAGS+= -DHAVE_CONFIG_H